Code-Division Multiple Access (CDMA) is a spread spectrum encoding method that enables many users to simultaneously transmit separate signals over the same spectral bandwidth. In CDMA, a data signal of bandwidth D is modulated by a higher rate coded waveform of bandwidth C. The resulting signal has a bandwidth of D+C, which, for large ratios of C to D is approximately equal to C. The ratio C/D is commonly referred to as the spreading ratio, the spreading gain, or the processing gain. The intended receiver modulates the received signal by an exact replica of the coded waveform to remove the code modulation and recover the data signal. The coded waveform may be any of many types but the primary one of interest here is a binary coded bi-phase modulation, also referred to as binary phase shift keying, or BPSK, modulation. The signaling rate of the coded spreading waveform is commonly called the chip rate.
The number of users that could occupy the same spreading bandwidth C is regulated by the processing gain of the high rate modulation, i.e., the ratio of the modulation rate to the data rate, C/D. In theory, this ratio is equal to the number of users. But in practice, due to the need to maintain low cross-correlation properties between the high rate sequences, the number of usable sequences, hence users, is somewhat less than the processing gain.
There has been considerable interest within the communications industry in recent years on the potential for Optical CDMA(OCDMA) to make more efficient use of the bandwidth available in fiber optic communications systems. The main problem with fiber optic systems is the inefficient nature of dedicated bandwidth allocation architectures. Many communications, particularly Internet Protocol communications, are extremely bursty. Therefore, as more users are added and depart, the bandwidth resource must be dynamically re-allocated. This may not be feasible.
The traditional method of signal processing used to address this problem in fiber optic systems is a frequency domain multiplexing protocol called wavelength division multiplexing (WDM). In WDM, the optical efficiency is increased by the creation of a plurality of wavelengths, each carrying a separate signal. Still, the number of wavelengths or channels that can be supported is constrained by the stability of each discrete wavelength and the tuning range of the diode laser. OCDMA is suggested as an alternative or in conjunction with WDM to increase the efficient use of fiber communications systems. The primary advantage of code division multiple access, as opposed to other optical multiple access or multipexing techniques, is the reduced requirement on coordination of exact timing and frequency allocations to the multiple users. In OCDMA, all of the users occupy the same time and frequency space and are precisely separated using their specific chipping code, a much simpler task.
Earlier inventions have been described to implement optical CDMA (OCDMA), which can be grouped in three categories: simple spectral domain methods, complex spectral domain methods, and time-domain based systems.
In a spectrally encoded OCDMA system, each user is identified by a particular pattern of spectral (frequency) components. These patterns can be encoded with a simple periodic optical filter, as disclosed by Pfeiffer in U.S. Pat. Nos. 5,784,506 and 6,215,573. In U.S. Pat. No. 5,784,506, Pfeiffer discloses an electronic decoding of the spectral encoded signal. In U.S. Pat. No. 6,215,573, Pfeiffer discloses an optical receiver with filtering characteristics for decreasing cross talk. In U.S. Pat. No. 5,867,290, Dutt et al. disclose a system whereby the spectrally encoded signal is created by selectively attenuating certain wavelengths from a broadband light source.
Typical OCDMA proposed systems use uni-polar codes that use plus ones (+1) and zeros (0), generally called on-off keying. Such codes are used because they are easily optically detected. This inherently reduces optical efficiency because a “0” code removes or discards available light. To increase optical efficiency, it is far better to use bi-polar codes, i.e. those consisting of plus one (+1) and minus one (−1). However, detection of bi-polar codes requires detection in the presence of an unmodulated reference beam, i.e., coherent detection. Coherent detection is difficult and expensive to achieve in a practical system.
The Dutt system is not very optically efficient due to the use of uni-polar codes and it has a limited code set. The Pfeiffer systems are more efficient but also have a limited code set resulting in a limited number of users.
In U.S. Pat. No. 5,760,941, Young et al. disclose a method and system for transmitting bi-polar codes using pairs of uni-polar codes. This method requires each of the pairs of uni-polar codes to be separately transmitted on separate fibers or opposite polarization.
In U.S. Pat. No. 6,236,483, Dutt et al. disclose a system based upon Young U.S. Pat. No. 5,760,941 with the addition of the use of sub-band encoding to divide the spectra into sub-groups.
Both the Dutt and Young systems are based upon attenuating the optical carrier using uni-polar codes. This scheme cannot achieve optimal optical efficiency.
In U.S. Pat. No. 6,313,771, Munroe et al. disclose an OCDMA system based upon use of fiber Bragg gratings to encode a short pulse into a sequence of plus one (+1) and minus one (−1) coded pulses, i.e., optically efficient bi-polar codes. In order to overcome inherent limitations of fiber Bragg gratings, this method specifically uses two stages of encoding to achieve a relatively long encoding pattern. This multi-stage system is complex to build and relies on two fiber Bragg gratings. This is less optically efficient than using a single grating.
In U.S. Pat. No. 6,292,282, Mossberg et al. disclose a time wavelength multiple access communication system whereby the optical signal of a user is separated into a small number of spectral bands. The resulting bands are transmitted in a specific time-sequence order. A decoder for a specific user removes the time sequencing of the spectral bands such that a signal from the intended user is time-aligned. The number of frequency bands, and hence, the number of available codes, and therefore users, is limited in a practical system.
In summary, the existing methods of OCDMA are not very efficient which yields a lower number of potential users. Furthermore, they rely upon grating technologies that have limited resolution. Lastly, some of the more efficient methods are complex and costly to manufacture.